Resistance of CD7-deficient mice to lipopolysaccharide-induced shock syndromes

Chimeric antigen receptor T-cell therapy for T-cell acute lymphoblastic leukemia

Chimeric antigen receptor (CAR) T-cell therapy is a new and effective treatment for patients with hematologic malignancies. Clinical responses to CAR T cells in leukemia, lymphoma, and multiple myeloma have provided strong evidence of the antitumor activity of these cells. In patients with refractory or relapsed B-cell acute lymphoblastic leukemia (ALL), the infusion of autologous anti-CD19 CAR T cells is rapidly gaining standard-of-care status and might eventually be incorporated into frontline treatment. In T-ALL, however, leukemic cells generally lack surface molecules recognized by established CAR, such as CD19 and CD22. Such deficiency is particularly important, as outcome is dismal for patients with T-ALL that is refractory to standard chemotherapy and/or hematopoietic stem cell transplant. Recently, CAR T-cell technologies directed against T-cell malignancies have been developed and are beginning to be tested clinically. The main technical obstacles stem from the fact that malignant and normal T cells share most surface antigens. Therefore, CAR T cells directed against T-ALL targets might be susceptible to self-elimination during manufacturing and/or have suboptimal activity after infusion. Moreover, removing leukemic cells that might be present in the cell source used for CAR T-cell manufacturing might be problematic. Finally, reconstitution of T cells and natural killer cells after CAR T-cell infusion might be impaired. In this article, we discuss potential targets for CAR T-cell therapy of T-ALL with an emphasis on CD7, and review CAR configurations as well as early clinical results.

CD7 protein plays a crucial role in T cell infiltration in tumors

CD7 protein as a target is being used to treat CD7⁺ lymphoma; however, the role of CD7 in the hematopoietic system remains largely unknown. Therefore, we evaluated the effects of CD7 KO in mice. The differentiation of the hematopoietic system in the bone marrow and the number of various cell types in the thymus and spleen did not differ between CD7 KO and WT mice. After subcutaneous inoculation of B16-F10 melanoma cells, tumors from CD7 KO mice grew more rapidly, and the proportion of CD8⁺ T cells in the spleen and tumors decreased. In vitro, the infiltration and adhesion of CD8⁺ T cells from the spleen of CD7 KO mice were weakened. Blocking CD7 in normal T cells did not alter the migration and infiltration, but in Jurkat, CCRF-CEM, and KG-1a tumor cell lines, migration and invasion were significantly reduced after blocking CD7. Therefore, CD7 does not affect hematopoietic system development but plays a crucial role in T cell infiltration into tumors.

CD7-deleted hematopoietic stem cells can restore immunity after CAR T cell therapy

Targeting T cell malignancies with universal CD7-targeting chimeric antigen receptor T cells (UCART7) can lead to profound immune deficiency due to loss of normal T and NK cells. While a small population of endogenous CD7- T cells exists, these cells are unlikely to be able to repopulate the entire immune repertoire after UCART7 treatment, as they are limited in number and proliferative capacity. To rescue T and NK cells after UCART7, we created hematopoietic stem cells genetically deleted for CD7 (CD7-KO HSCs). CD7-KO HSCs were able to engraft immunodeficient mice and differentiate into T and NK cells lacking CD7 expression. CD7-KO T and NK cells could perform effector functions as robustly as control T and NK cells. Furthermore, CD7-KO T cells were phenotypically and functionally distinct from endogenous CD7- T cells, indicating that CD7-KO T cells can supplement immune functions lacking in CD7- T cells. Mice engrafted with CD7-KO HSCs maintained T and NK cell numbers after UCART7 treatment, while these were significantly decreased in control mice. These studies support the development of CD7-KO HSCs to augment host immunity in patients with T cell malignancies after UCART7 treatment.

Molecular cloning and expression analysis of pig CD7

CD7 is an integral membrane protein which mediates an important signal to mediate the differentiation, activation, and regulation of some T cells and NK cells. However, only human and mouse CD7 have been identified and studied among mammalian species. In this study, we cloned pig CD7 cDNA and determined its complete cDNA sequence. Pig CD7 cDNA contained an open reading frame (627 bp) encoding 208 amino acids with well conserved motifs involved in signal transduction within cytoplasmic tail among mammalian species. Pig CD7 mRNA was detected by RT-PCR in mainly lymphoid tissues, indicating the conserved functions of CD7 in pigs. Moreover, we generated soluble pig CD7 fusion immunoglobulin (pig CD7Ig) containing extracellular domain of pig CD7 to test whether pig CD7 binds to pig galectin-3. Flow cytometry and immunohistochemistry analyses indicated that soluble pig CD7Ig can bind to galectin-3 expressed in macrophages and epithelial cells of small intestine. These results help to analyze the structural relationship between CD7 and its ligand transferring signal transduction among mammalian species.

Autocrine GM-CSF transcription in the leukemic progenitor cell line KG1a is mediated by the transcription factor ETS1 and is negatively regulated through SECTM1 mediated ligation of CD7

BACKGROUND

CD7 expression is found on ~30% of acute myeloblastic leukemias (AML). The leukemic progenitor cell line KG1a (CD7+) constitutively expresses GM-CSF while the parental KG1 (CD7-) cell line does not. This study focuses on the molecular basis of CD7 mediated GM-CSF regulation.

METHODS

KG1a cells were treated with recombinant SECTM1-Fc protein, the PI3K kinase inhibitors wortmannin, LY292004, or PI4K activator spermine. Stable KG1-CD7+, KG1a-shCD7, KG1a-shETS1 as well as KG1a-GFP, KG1a-PKCβII-GFP cell lines were generated and the levels of CD7, GM-CSF and ETS-1 mRNA and protein were compared by real-time-PCR, western blotting, flow cytometry and ELISA.

RESULTS

SECTM1 is expressed in Human Bone Marrow Endothelial Cells (HBMEC) and its expression can be upregulated by both IFN-γ. KG1a cells demonstrated high expression levels of CD7 and ETS-1 allowing a constitutative signaling through the PI3K/Atk pathway to promote GM-CSF expression, while KG1 cells with low expression of CD7 and ETS-1 showed low GM-CSF expression. On KG1a cells GM-CSF expression could be negatively regulated by PI3K inhibitors or by recombinant SECTM1-Fc. Overexpression of CD7 in KG1 cells was insufficient to promote GM-CSF expression, while silencing of CD7 or ETS-1 resulted in reduced GM-CSF expression levels. Differentiation capable KG1a cells overexpressing PKCβII illustrated complete loss of CD7, but maintained normal levels of both ETS-1 and GM-CSF expression.

CONCLUSION

These findings add an additional layer to the previously described autocrine/paracrine signaling between leukemic progenitor cells and the bone marrow microenvironment and highlight a role for SECTM1 in both normal and malignant hematopoiesis.

GENERAL SIGNIFICANCE

This work shows that SECTM1 secreted from bone marrow stromal cells may interact with CD7 to influence GM-CSF expression in leukemic cells.

The signaling tool box for tyrosine-based costimulation of lymphocytes

Triggering lymphocyte effector functions is controlled by a diverse array of immune cell coreceptors that dampen or potentiate the primary activation signal from antigen receptors. Attenuation of lymphocyte activation has been shown to be accomplished by immunoreceptor tyrosine-based inhibition motifs that upon phosphorylation recruit protein or lipid phosphatases. By contrast, a general concept of signal amplification and/or diversification is still out. However, the recent discovery of antigen receptor-intrinsic costimulation by membrane-bound immunoglobulins in class-switched memory B cells identified a consensus phosphorylation motif that can boost antigen-induced signal chains and is also employed by costimulatory receptors on T and Natural Killer cells to provide secondary signals for cellular activation. Here we define a common basis of tyrosine-based lymphocyte costimulation comprising immunoglobulin tail tyrosine (ITT)-like phosphorylation motifs and their proximal effectors, growth factor receptor-bound protein (Grb) 2 and phosphatidylinositol-3 kinase (PI3K) enzymes of class IA.

Matrix metalloproteinases as drug targets in infections caused by gram-negative bacteria and in septic shock

The mammalian immune system is optimized to cope effectively with the constant threat of pathogens. However, when the immune system overreacts, sepsis, severe sepsis, or septic shock can develop. Despite extensive research, these conditions remain the leading cause of death in intensive care units. The matrix metalloproteinases (MMPs) constitute a family of proteases that are expressed in developmental, physiological, and pathological processes and also in response to infections. Studies using MMP inhibitors and MMP knockout mice indicate that MMPs play essential roles in infection and in the host defense against infection. This review provides a brief introduction to some basic concepts of infections caused by gram-negative bacteria and reviews reports describing MMP expression and inhibition, as well as studies with MMP-deficient mice in models of infection caused by gram-negative bacteria and of septic shock. We discuss whether MMPs should be considered novel drug targets in infection and septic shock.

The RON receptor tyrosine kinase regulates IFN-gamma production and responses in innate immunity

Receptor tyrosine kinases are emerging as a class of key regulators of innate immune responses. We have shown previously that the RON receptor tyrosine kinases (murine Stk), expressed on tissue-resident macrophages, inhibit classical macrophage activation while promoting hallmarks of alternative activation, thus regulating the critical balance between the inflammatory and wound-healing properties of activated macrophages. We have also shown previously that RON(-/-) mice are more susceptible to in vivo endotoxin challenge than wild-type mice, suggesting that the expression of this receptor confers a degree of endotoxin resistance to these animals. Here we demonstrate that, in response to in vivo LPS challenge, RON(-/-) mice harbor significantly increased systemic levels of IFN-gamma and IL-12p70 and increased levels of IL-12p40 transcript in their spleen. This elevation of IFN-gamma can be attributed to splenic NK cells responding to the elevated levels of IL-12. Analysis of RON and IFN-gamma receptor double-knockout mice indicates that the enhanced susceptibility of RON(-/-) mice to endotoxin challenge is dependent on IFN-gamma-mediated signals. In vitro studies demonstrate that stimulation of primary peritoneal macrophages with macrophage-stimulating protein, the ligand for RON, inhibits IFN-gamma-induced STAT1 phosphorylation and CIITA expression, resulting in reduced surface levels of MHC class II. Further studies demonstrating the induction of suppressor of cytokine signaling 1 via macrophage-stimulating protein/RON signaling provide a potential mechanistic insight into this regulatory pathway. These results indicate that the RON receptor regulates both the production of and response to IFN-gamma, resulting in enhanced susceptibility to endotoxin challenge.

Murine CD7 shares antigenic cross-reactivity with HSP-60

Human (h) CD7 is a 40 kDa single chain Ig superfamily molecule that is expressed on thymocytes, a major subunit of peripheral T cells, and most natural killer cells. Ligands for hCD7 include the epithelial cell-produced molecule, K-12, and galectin. Mice deficient in CD7 have been shown to be resistant to LPS-induced endotoxic shock syndromes. However, monoclonal antibodies (MAb) to mouse (m) CD7 have yet to be produced, nor is the distribution of mCD7 protein in mice known. We have raised a panel of three rat MAbs to mCD7 by immunizing rats with recombinant mCD7 protein. However, using Western blot and immunoprecipitation of tissue extracts from mouse thymus, spleen, liver, brain, lymph node and skin, these anti-mouse CD7 MAbs bound only to murine heat shock protein 60 (HSP-60) present both in wild-type (CD7+/+) and CD7-deficient (CD7-/-) mice. Epitope mapping of the sites on HSP-60 and recombinant mCD7 recognized by mCD7 MAbs demonstrated non-homologous amino acid sequence epitopes recognized by anti-CD7 MAbs on both proteins. These data demonstrated molecular mimicry of mCD7 with HSP-60, and leave open the question of surface expression of mCD7.

Expression of the CD7 ligand K-12 in human thymic epithelial cells: regulation by IFN-gamma

CD7 is an immunoglobulin superfamily molecule expressed on T, NK, and pre-B lymphocytes. Previous studies have demonstrated a role for CD7 in T- and NK-cell activation and cytokine production. Recently, an epithelial cell secreted protein, K12, was identified as a CD7 ligand. Although CD7 is expressed intrathymically, it is not known if K12 is produced in human thymus. To determine roles that K12 might play in the human thymus, we analyzed expression of K12 in human thymocytes, thymic epithelial cells (TE), and thymic fibroblasts. We found that recombinant human K12 bound strongly to soluble hCD7, with a Keq of 37.6x10(-9) M, and this interaction was inhibited by a novel antihuman K12 monoclonal antibody (K12-A1). K12 mRNA was detected by RT-PCR and northern analysis in human TE and thymic fibroblasts, but not in human thymocytes. Expression of K12 in TE cells was upregulated by IFN-gamma. Taken together, these data demonstrated that K12 is produced by human TE cells and thymic fibroblasts, and is regulated in thymus by IFN-gamma. These data suggest a role for thymic microenvironment-produced K12 in regulation of thymocyte signaling and cytokine release, particularly in the setting of thymus pathology where IFN-gamma is upregulated such as myasthenia gravis.

CD7 and CD28 Are Required for Murine CD4+CD25+ Regulatory T Cell Homeostasis and Prevention of Thyroiditis

CD7 and CD28 are T cell Ig superfamily molecules that share common signaling mechanisms. To determine roles CD7 and CD28 might play in peripheral lymphocyte development and function, we have generated CD7/CD28-double-deficient mice. CD7- and CD28-single-deficient and CD7/CD28-double-deficient mice had normal levels of CD4 and CD8-single-positive T cells in thymus and spleen. However, CD28-deficient mice had decreased CD4+CD25+ T cells in spleen compared with wild-type mice, and CD7/CD28-double-deficient mice had decreased numbers of CD4+CD25+ T cells in both thymus and spleen compared with both wild-type and CD28-deficient mice. Functional studies demonstrated that CD4+CD25+ T cells from CD28-deficient and CD7/CD28-double-deficient mice could mediate suppression of CD3 mAb activation of CD4+CD25- wild-type T cells, but were less potent than wild-type CD4+CD25+ T regulatory cells. Thyroiditis developed in aged CD7/CD28-double-deficient mice (>1 year) that was not seen in age-matched control mice or single CD7- or CD28-deficient mice, thus suggesting in vivo loss of T regulatory cells allowed for the development of spontaneous thyroiditis. Taken together, these data demonstrated collaborative roles for both CD7 and CD28 in determination of number and function of CD4+CD25+ T regulatory cells in the thymus and peripheral immune sites and in the development of spontaneous thyroiditis.

Bromelain treatment alters leukocyte expression of cell surface molecules involved in cellular adhesion and activation

Bromelain is a natural proteinase preparation derived from pineapple stem that is marketed for oral use as a digestive aid and as an antiinflammatory agent. Bromelain treatment in vitro has been previously shown to selectively remove certain cell surface molecules that may affect lymphocyte migration and activation. This study reports the effects of bromelain on a broad range of cell surface molecules and on lymphocytes, monocytes, and granulocytes under physiologically relevant conditions. In vitro bromelain treatment of leukocytes in whole blood proteolytically altered 14 of 59 leukocyte markers studied. Constitutively expressed bromelain-sensitive molecules included CD7, CD8alpha, CD14, CD16, CD21, CD41, CD42a, CD44, CD45RA, CD48, CD57, CD62L, CD128a, and CD128b. The proteolytic effect of bromelain increased as the concentration of plasma decreased, with EC50 ranging from >1000 microg/ml for 100% plasma to approximately 1 microg/ml in the absence of plasma, indicating the presence of an inhibitor of bromelain in plasma. alpha2-macroglobulin purified from plasma mimicked the inhibitory effect of whole plasma on bromelain activity. If proteolysis is required for the antiinflammatory actions of oral bromelain, these data suggest that the required concentrations are more likely to be achieved locally in the gastrointestinal tract or in other tissue sites where the plasma concentration is low, rather than in the bloodstream. The cell surface molecules altered by bromelain are involved in leukocyte homing and cellular adhesion and activation. Thus bromelain could potentially exert an antiinflammatory effect by multiple mechanisms, including alterations in leukocyte migration and activation.

Critical role of NK cells rather than V alpha 14(+)NKT cells in lipopolysaccharide-induced lethal shock in mice

Although macrophages play a central role in the pathogenesis of septic shock, NK1(+) cells have also been implicated. NK1(+) cells comprise two major populations, namely NK cells and V alpha 14(+)NKT cells. To assess the relative contributions of these NK1(+) cells to LPS-induced shock, we compared the susceptibility to LPS-induced shock of beta(2)-microglobulin (beta(2)m)(-/-) mice that are devoid of V alpha 14(+)NKT cells, but not NK cells, with that of wild-type (WT) mice. The results show that beta(2)m(-/-) mice were more susceptible to LPS-induced shock than WT mice. Serum levels of IFN-gamma following LPS challenge were significantly higher in beta(2)m(-/-) mice, and endogenous IFN-gamma neutralization or in vivo depletion of NK1(+) cells rescued beta(2)m(-/-) mice from lethal effects of LPS. Intracellular cytokine staining revealed that NK cells were major IFN-gamma producers. The J alpha 281(-/-) mice that are exclusively devoid of V alpha 14(+)NKT cells were slightly more susceptible to LPS-induced shock than heterozygous littermates. Hence, LPS-induced shock can be induced in the absence of V alpha 14(+)NKT cells and IFN-gamma from NK cells is involved in this mechanism. In WT mice, hierarchic contribution of different cell populations appears likely.

Biology of natural killer cells in cancer and infection

NK cells are the important cells of the immune system derived from stem cells in the marrow. Their physiology is tightly regulated to control proliferation, cytotoxicity and cytokine production. In cancer, NK cells may be abnormal due to the cancer itself or possibly related to its therapy. The finding of class I recognizing inhibitory receptors may play a role in stem cell transplant rejection, immune surveillance and cancer immunotherapy. NK cells should no longer be thought of as direct cytotoxic killers alone, as they clearly play a critical role in cytokine production which may be important to control cancer and infection. Understanding NK cell function and homing may lead to novel therapeutic strategies for the treatment of human disease.

T cell signal transduction and the role of CD7 in costimulation

The complex cellular interactions that govern the mammalian immune response are now known to include specific receptor/ligand interactions, recruitment of intracellular signaling molecules, activation of both kinases and phosphatases, and redistribution of macromolecular complexes into specific subcellular membrane locations that, in aggregate, result in transcriptional activation. While the TCR-CD3 signal is critical for activation of the resting T cell, it alone is not sufficient to initiate transcriptional activation or generate an effective immune response. A number of other coreceptor molecules, including CD4, CD8, and CD28, have now been characterized that also play important roles in initiating or amplifying the activation of the T cell. A 40 kDa member of the immunoglobulin superfamily, the CD7 molecule, has also been shown to have costimulatory activity and to induce tyrosine and lipid kinase activities. Here we will review the signaling pathways initiated by TCR, CD28, and CD7, as well as the functional consequences of signal transduction through these receptors.

Comparison of thymocyte development and cytokine production in CD7-deficient, CD28-deficient and CD7/CD28 double-deficient mice

CD7 and CD28 are Ig superfamily molecules expressed on thymocytes and mature T cells that share common signaling 0mechanisms and are co-mitogens for T cell activation. CD7-deficient mice are resistant to lipopolysaccharide (LPS)-induced shock syndrome, and have diminished in vivo LPS-triggered IFN-gamma and tumor necrosis factor (TNF)-alpha production. CD28-deficient mice have decreased serum Ig levels, defective IgG isotype switching, decreased T cell IL-2 production and are resistant to Staphylococcus aureus enterotoxin-induced shock. To determine synergistic roles CD7 and CD28 might play in thymocyte development and function, we have generated and characterized CD7/CD28 double-deficient mice. CD7/CD28-deficient mice were healthy, reproduced normally, had normal numbers of thymocyte subsets and had normal thymus histology. Anti-CD3 mAb induced similar levels of apoptosis in CD7-deficient, CD28-deficient and CD7/CD28 double-deficient thymocytes as in control C57BL/6 mice (P = NS). Similarly, thymocyte viability, apoptosis and necrosis following ionomycin or dexamethasone treatment were the same in control, CD7-deficient, CD28-deficient and CD7/CD28-deficient mice. CD28-deficient and CD7/CD28-deficient thymocytes had decreased [3H]thymidine incorporation responses to concanavalin A (Con A) stimulation compared to control mice (P < or = 0.01 and P < or = 0.05 respectively). CD7/CD28 double-deficient mice had significantly reduced numbers of B7-1/B7-2 double-positive cells compared to freshly isolated wild-type, CD7-deficient and CD28-deficient thymocytes. Con A-stimulated CD4/CD8 double-negative (DN) thymocytes from CD7/CD28 double-deficient mice expressed significantly lower levels of CD25 when compared to CD4/CD8 DN thymocytes from wild-type, CD7-deficient and CD28-deficient mice (P < 0.05). Anti-CD3-triggered CD7/CD28-deficient thymocytes also had decreased IFN-gamma and TNF-alpha production compared to C57BL/6 control, CD7-deficient and CD28-deficient mice (P < or = 0.05). Thus, CD7 and CD28 deficiencies combined to produce abnormalities in the absolute number of B7-1/B7-2-expressing cells in the thymus, thymocyte IL-2 receptor expression and CD3-triggered cytokine production.

Resistance of natural killer T cell-deficient mice to systemic Shwartzman reaction

The generalized Shwartzman reaction in mice which had been primed and challenged with lipopolysaccharide (LPS) depends on interleukin (IL)-12-induced interferon (IFN)-gamma production at the priming stage. We examined the involvement in the priming mechanism of the unique population of Valpha14 natural killer T (NKT) cells because they promptly produce IFN-gamma after IL-12 stimulation. We report here that LPS- or IL-12-primed NKT cell genetically deficient mice were found to be resistant to LPS-elicited mortality. This outcome can be attributed to the reduction of IFN-gamma production, because injection of recombinant mouse IFN-gamma, but not injection of IL-12, effectively primed the NKT cell-deficient mice. However, priming with high doses of LPS caused mortality of severe combined immunodeficiency, NKT cell-deficient, and CD1-deficient mice, indicating a major contribution of NKT cells to the Shwartzman reaction elicited by low doses of LPS, whereas at higher doses of LPS NK cells play a prominent role. These results suggest that the numerically small NKT cell population of normal mice apparently plays a mandatory role in the priming stage of the generalized Shwartzman reaction.

CD1 molecules and CD1-dependent T cells in bacterial infections: a link from innate to acquired immunity?

The MHC class I-like, non-polymorphic CD1 molecules represent a novel system for the presentation of glycolipid antigens to T lymphocytes. CD1-mediated T cell responses appear to play distinct roles during bacterial infections such as in tuberculosis. This review deals with two aspects of CD1-mediated immune reactions. First we discuss the role of group II CD1-dependent NK T cells in bacterial infection. Second, we provide an insight into differential intracellular meeting points for antigen processing between group I CD1 molecules, mycobacteria and mycobacterial glycolipid antigens.

CD1 and CD1-restricted T cells in infections with intracellular bacteria

Glycolipid-specific, CD1a-, b- and c-dependent cytotoxic T cells have recently been shown to be involved in the host response against tuberculosis. These CD1 molecules 'sample' mycobacterial glycolipids from different intracellular sites in the infected cell. Additionally, upon microbial encounter, CD1d-dependent natural killer T cells promptly produce cytokines and perform regulatory activities. Here, we discuss the intracellular localization of CD1 molecules and mycobacterial lipids and the role of CD1-mediated T-cell responses in mycobacterial infections.

Lipopolysaccharide and D-galactosamine-induced hepatic injury is mediated by TNF-alpha and not by Fas ligand

Tumor necrosis factor (TNF)-alpha and Fas ligand (FasL) are trimeric proteins that induce apoptosis through similar caspase-dependent pathways. Hepatocytes are particularly sensitive to inflammation-induced programmed cell death, although the contribution of TNF-alpha and/or FasL to this injury response is still unclear. Here, we report that D-galactosamine and lipopolysaccharide-induced liver injury in C57BL/6 mice is associated with increased hepatic expression of both TNF-alpha and FasL mRNA. Pretreatment of mice with a TNF-binding protein improved survival, reduced plasma aspartate aminotransferase concentrations, and attenuated the apoptotic liver injury, as determined histologically and by in situ 3' OH end labeling of fragmented nuclear DNA. In contrast, pretreatment of mice with a murine-soluble Fas fusion protein (Fasfp) had only minimal effect on survival, and apoptotic liver injury was either unaffected or exacerbated depending on the dose of Fasfp employed. Similarly, mice with a spontaneous mutation in FasL (B6Smn.C3H-Fasl(gld) derived from C57BL/6) were equally sensitive to D-galactosamine/lipopolysaccharide-induced shock. We conclude that the shock and apoptotic liver injury after D-galactosamine/lipopolysaccharide treatment are due primarily to TNF-alpha release, whereas increased FasL expression appears to contribute little to the mortality and hepatic injury.

Identification of CD7 as a cognate of the human K12 (SECTM1) protein

CD7 is a 40-kDa protein found primarily on T, NK, and pre-B cells; the function of the CD7 protein in the immune system is largely unknown. The K12 (SECTM1) protein was originally identified by its location just upstream of the CD7 locus. The K12 gene encodes a transmembrane protein of unknown function. In order to clone a K12-binding protein, we generated a soluble version of the human K12 protein by fusing its extracellular domain to the Fc portion of human IgG(1). Flow cytometry experiments showed that the K12-Fc fusion protein bound at high levels to both human T and NK cells. Precipitation experiments using K12-Fc on (35)S-radiolabeled NK cells lysates indicated that the K12 cognate was an approximately 40-kDa protein. A human peripheral blood T cell cDNA expression library was screened with the K12-Fc protein, and two independent, positive cDNA clones were identified and sequenced. Both cDNAs encoded the same protein, which was CD7. Thus, K12 and CD7 are cognate proteins that are located next to each other on human chromosome 17q25. Additionally, we have cloned the gene encoding the mouse homologue of K12, shown that it maps near the mouse CD7 gene on chromosome 11, and established that the mouse K12 protein binds to mouse, but not human, CD7. Mouse K12-Fc inhibited in a dose-dependent manner concanavalin A-induced proliferation, but not anti-TcRalpha/beta induced proliferation, of mouse lymph node T cells. Human K12-Fc stimulated the up-regulation of CD25, CD54, and CD69 on human NK cells in vitro.